MR-1 RPM

[Matt Phillips]

Altitude does play a difference on the non supercharged engines. Up here at Lake Tahoe (6300 feet) my MR1s will top out at about 9000. Down at lake Shasta (about 1000 feet) they will top out close to 10,000. Down on the delta, a touch more than 10,000. If you're getting close to 10k out of them...like you said, you're likely splitting hairs.

 

[ncnmra]

For fun, I did some non-scientific math. The Air density at sea level with given temperature, pressure, dew point was 0.0727 lb/ft^3. With the other variables staying equal but at 580' (Lake Huron), it is 0.0712lb/ft^3. That represents 98% of the density at sea level. Now, this is wonky math perhaps, and I certainly don't know if the relationship is linear to RPM, but it certainly closely matches my observations.

I'm still going to replace the plugs on the port side since I'm a little perplexed why the engine response isn't identical. By @buckbuck 's response, it sounds like both engines should be spinning up identically.

 

[Speedling]

For fun, I did some non-scientific math. The Air density at sea level with given temperature, pressure, dew point was 0.0727 lb/ft^3. With the other variables staying equal but at 580' (Lake Huron), it is 0.0712lb/ft^3. That represents 98% of the density at sea level. Now, this is wonky math perhaps, and I certainly don't know if the relationship is linear to RPM, but it certainly closely matches my observations.

I'm still going to replace the plugs on the port side since I'm a little perplexed why the engine response isn't identical. By @buckbuck 's response, it sounds like both engines should be spinning up identically.

Incorrect. Pump loading is a factor.
Not going to take credit for others work so look it up here: http://www.groupk.com/yjetboat.htm

Very good read for any jet boater looking for performance of any kind.

 

[ncnmra]

Incorrect. Pump loading is a factor.
Not going to take credit for others work so look it up here: http://www.groupk.com/yjetboat.htm

Very good read for any jet boater looking for performance of any kind.

Which part is incorrect? I understand that the impellers are supposed to be different on port/starboard, but I assumed that is to make the throttle response the same.

You have basically the same boat: do your engines throttle up exactly at the same rate?

 

[Speedling]

If your impellers are exactly the same pitch then they will differ in rpms.
Here is the research from groupk:
Twin Engine Pump Loading

The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines.

Getting past the whole theoretical aspect, the functional result is what’s important.. Because of the phenomenon described above, the right side pump is generating more actual thrust than the left pump at any given rpm. This causes the steering wheel to constantly be “tugging” toward a left hand turn, and at the same time causes the left side engine to rev higher than the right engine in order to generate the same thrust.

After weeks of testing, we eventually resolved all these problems (on twin motor Yamahas) by fitting better design impellers on the pumps, and applying a noticeably steeper pitch to the left hand side prop. The end result is engines that turn virtually identical rpms all the way through the throttle movement range, and an end to the steering wheel constantly “tugging” to the left. In most cases, the “off-the-shelf” pitch of the props we chose were not right on the money, so we custom pitch each pair that we sell to the specs that worked best during our on-water tests. Installing these “staggered pitch” aftermarket impellers is one of the most effective improvements that you can make to your twin motor Yamaha jet-boat.
______
My rpms on my boat are different from each other and i have solas 14/20 impellers.
My engines do exactly what thier research saw. Port side higher rpms by 200 rpm on mine.
I am 9800/9600.

 

[ncnmra]

@Speedling : yup; I get all that. But I have stock impellers and I notice the opposite; port is lower than starboard by ~200 rpm.

 

[Speedling]

http://www.greenhulk.net/forums/showthread.php?t=2614
Impeller bending is not for the weak of heart but a tweak is all you need to dial that in. If you are that close on rpm and speed is decent then minor tweaks gain minor results.

 

[swatski]

@Speedling : yup; I get all that. But I have stock impellers and I notice the opposite; port is lower than starboard by ~200 rpm.

I'm sure you thought of it but just to be sure - the port side impeller has the "notches" in the neck, someone here had found theirs swapped wrong way/side from factory.

--

 

[ncnmra]

Can they be identified without pulling the pump?

 

[Speedling]

Hardly.
Pulling the pump is easy. You will chuckle to the fact that it is something you wanted to avoid.

 

[ncnmra]

Hardly.
Pulling the pump is easy. You will chuckle to the fact that it is something you wanted to avoid.

Except for @Big Shasta 's case LOL

 

[swatski]

Can they be identified without pulling the pump?

Yes. Top post from @bronze_10. https://jetboaters.net/threads/impellers.9163/#post-157009

Just to avoid confusion:
this is port

And starboard:

 

[tdonoughue]

Cool! And the 'notches' are the lines on the hex part right before the blades? I can actually see it...

Not having any issues, but about to head up to the boat to continue gelcoat repair and probably throw some silicone back there that has disappeared. Will need to look at this just for fun.

 

[ncnmra]

I found a really interesting online calculator that relates Density Altitude (based on altitude, air temperature, air pressure, humidity) to horsepower. I wonder what the HP to rpm correlation for our engines may be in this case:

http://wahiduddin.net/calc/calc_hp_dp.htm

 

[ncnmra]

Small update here:

1. I changed the plugs and made sure the gap was correct (oddly, the wire gap tool showed a different gap than the slide tool so I split the difference).
2. I confirmed that the port impeller has the notches, and the starboard does NOT.
3. I have cleaned out the spark arrestor (ribbon?) on the port engine (carb cleaner)

The changes did not make much difference.

I pulled my engine hours using YDS and found something very peculiar...check out the engine hours on port vs starboard:



What I find odd is that the port engine is only showing 0.14 hours from 10000+, vs 2.6h for the starboard. That means that in its LIFETIME (> 200h), it has hit 10000+rpm for only 8.4 minutes, vs the 2.6 hours on the starboard.

This indicates to me that it has always been this way: but I guess the question is "why" and "should I care". I plan on doing a compression test on both engines to be sure, but I'm sort of thinking that it's just the way it is/has been/will be. I'm the 3rd owner, so I don't know any prior history.

The engines always start with zero issues, idle and rev well and the boat feels like it has a good hole shot (can't really compare to another identical boat). My working theory is that my slightly higher elevation and humidity means that my rpms will always run a little lower, and the starboard side is compensated by the lesser pitch of the factory impeller. Interestingly the starboard side doesn't get over 10000rpm easily either, although it does seem to get up to top revs a little quicker.

I can hit about 50mph with 1/2 tank of gas, two people, two batteries, tower speakers, amp, a heavy anchor and a bunch of other weight.

Any other thoughts?

 

[Hannibal]

Man! I just read the timing chain thread. Now reading of 10k+rpm. I've always thought of 4-5k rpm as pushing it with a gas engine. Just yesterday we were cruising the Gulf in our new FSH at 6k rpm while whispering to myself "stay together cheeks".

 

[tdonoughue]

@ncnmra another thought:

Perhaps the throttles were out of adjustment (or still are) and therefore would not make it up to 10k on the one engine until just recently when they were adjusted (or they fell out of adjustment shortly after delivery and still are out of adjustment)?

 

[Big Shasta]

Man! I just read the timing chain thread. Now reading of 10k+rpm. I've always thought of 4-5k rpm as pushing it with a gas engine. Just yesterday we were cruising the Gulf in our new FSH at 6k rpm while whispering to myself "stay together cheeks".

The Yamaha street bike version of the 1000cc engine in these boats put out max HP at 12K+ RPM

 

[Hannibal]

The Yamaha street bike version of the 1000cc engine in these boats put out max HP at 12K+ RPM

Yeah it's a perception thing. My old Harley would launch confetti at half that rpm. My ol' Dodge Cummins would recycle itself at 4200rpm. I have to get my head wrapped around this new to me normal operating rpm.

 

[2nazt]

@Hannibal the MR-1 operates a lot higher than the 1.8. Our best cruise is almost redline to the 1.8